Calmodulin-stimulated phosphorylation of 17 beta-estradiol receptor on tyrosine.

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Abstract
The calf uterine 17.beta.-estradiol receptor is a phosphoprotein. Phosphorylation-dephosphorylation of the receptor is controlled by a cytosol receptor kinase that activates the hormone binding and by a nuclear phosphatase that inactivated this binding. The nature of the 17.beta.-estradiol receptor kinase was investigated. Highly purified calf uterus 17.beta.-estradiol receptor preinactivated by the nuclear phosphatase was used as substrate of the purified receptor kinase. Ca2+ and calmodulin stimulate both the kinase-dependent activation of the hormone binding and 32P incorporation from [.gamma.-32P]-ATP into the receptor. Maximal stimulation of hormone binding activation requires 1 .mu.M Ca2+ and 0.6 .mu.M calmodulin. Fifteen micromolar trifluoperazine is the lowest concentration that will prevent completely Ca2+-calmodulin stimulation of the kinase. The receptor is phosphorylated by the receptor kinase exclusively on tyrosine. Phosphorylation of proteins on tyrosine is a rare event implicated in hormone-induced cell growth and cell transformation.